The present invention relates generally to liquid crystals and their use in coating applications.
Liquid crystals include a large number of compositions which, in a liquid state, display properties typically observed in crystalline solids. The present invention is concerned primarily with liquid crystals displaying optical anisotropies, for example, those which undergo an appearance (e.g., color) change in response to an external stimulus such as a temperature change.
This particular class of liquid crystals is well-known to those skilled in the art and comprises, for example, various chiral nematic cholesterol and biphenyl derivatives. For further details about this type of liquid crystal, reference may be had to the following literature which is incorporated by reference herein for all purposes: H. Finkelmann and G. Rehage. "Investigation on Liquid Crystalline Polysiloxanes. 1 - Synthesis and Characterization of Linear Polymers." Makromol. Chem., Rapid Commun., 1 (1980). pp. 31-34., G. Rehage. "Flussigkristalline Polymers."Nachr. Chem. Tech. Lab., 32. No. 4 (1984). pp. 287-95."Flussigkristalle." Ullmanns Encylklopadie der Technischen Chemie (1976).
The use of these liquid crystals in coating applications is also generally known. See. e.g., D. Makow. "Color Gamut of Liquid Crystal Polysiloxanes." Mol. Cryst. Liq. Cryst., 123 (1985). pp. 341-53. also incorporated by reference herein for all purposes.
A typical application method for such liquid crystals involves: (1) first dissolving a liquid crystalline material (liquid crystal and/or precursor thereof) in an appropriate solvent.. (2) next applying the resulting solution as a thin layer (typically less than 10 .mu.m) onto a substrate: then (3) repeatedly and successively heating (often at temperatures of 80.degree. C. or higher) and cooling the thin layer to orient and fix the liquid crystal. Steps (2) and (3) must be repeated until the desired total layer thickness. liquid crystal orientation and appearance uniformity have been achieved.
This process has several disadvantages. For one, it is much too involved and time consuming for most commercial applications. Further, the repetitive heating in step (3) can affect the stability of the liquid crystalline materials and, consequently, the properties of the resulting liquid crystal. Also, even by careful control it is difficult, at best, with the above method to obtain a liquid crystal coating with a uniform appearance and effect.
It is, therefore, an object of the present invention to provide a vehicle by which liquid crystals may be more easily applied and readily used in coating applications.